Cermet-type alloy coating on metal base

ABSTRACT

AN IMPROVED CERMET-TYPE ALLOY AND METHOD OF MAKING SAME WHICH IS PARTICULARLY ADAPTABLE FOR FORMING PROTECTIVE SURFACE COATINGS ON HEAT-RESISTANT ALLOYS. A PARTICULATED MIXTURE IS FORMED CONTAINING TITANIUM AND/OR ZIRCONIUM REACTIVE METAL CONSTITUENTS THAT UNDERGO AN EXOTHERMIC REACTION UPON FUSION AT AN ELEVATED TEMPERATURE IN A SUBSTANTIALLY INERT ATMOSPHERE WITH SILICON AND/OR BORON PRESENT IN THE MIXTURE FORMING THE CORRESPONDING SILICIDES OR BORIDES OF THE REACTED METALS IN SITU WHICH ARE SUBSEQUENTLY PRECIPITATED AS A UNIFORMLY DISPERSED DISCONTINUOUS PHASE IN A CONTINUOUS PHASE OF A NICKEL AND/OR COBALT BASE MATRIX. THE INVENTION ALSO ENCOMPASSES NOVEL POWDER COMPOSITIONS FOR EXOTHERMICALLY FORMING THE CERMET-TYPE ALLOYS AND COATINGS. D R A W I N G

June 27, 1972 BREDZS ETAL 3,672,849

CERMET-TYPE ALLOY COATING 0N METAL BASE 'Original Filed Feb. 19, 1969 2Sheets-Sheet 1 Jun 27, 1972 zs ETAL 3572,84

CERMET-TYPE ALLOY COATING ON METAL BASE 'QriginaZL Filed Feb. 19. 1969 2Skits-SHOW! United States Patent Oflice 3,672,849 CERMET-TYPE ALLOYCOATING ON METAL BASE Nikolajs Bredzs, Detroit, and Forbes M. Miller,Dearborn, Mich., assignors to Wall Colmonoy Corporation Application Feb.19, 1969, Ser. No. 800,540, now Patent No. 3,547,673, dated Dec. 15,1970, which is a continuation-in-part of application Ser. No. 646,654,June 16, 1967. Divided and this application July 7, 1969, Ser. No.871,121 The portion of the term of the patent subsequent to Dec. 15,1987, has been disclaimed Int. Cl. B32b 15/00 U.S. Cl. 29-195 3 ClaimsABSTRACT OF THE DISCLOSURE An improved cermet-type alloy and method ofmaking same which is particularly adaptable for forming protectivesurface coatings on heat-resistant alloys. A particulated mixture isformed containing titanium and/or zirconium reactive metal constituentsthat undergo an exothermic reaction upon fusion at an elevatedtemperature in a substantially inert atmosphere with silicon and/orboron present in the mixture forming the corresponding silicides orborides of the reacted metals in situ which are subsequentlyprecipitated as a uniformly dispersed discontinuous phase in acontinuous phase of a nickel and/ or cobalt :base matrix. The inventionalso encompasses novel powder compositions for exothermically formingthe cermet-type alloys and coatings.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a divisionalapplication of copending application Ser. No. 800,540, filed Feb. 19,1969', Pat. 3,547,673, dated Dec. 15, 1970, which comprises acontinuation-impart application of application Ser. No. 646,654, filedJune 16, 1967, for Metallic Surface Coating and Method of Making Same,now abandoned.

BACKGROUND OF THE INVENTION Various coating compositions andcoatingtechniques have heretofore been used or proposed for use for providing aprotective coating on metal alloys which them selves are characterizedas heat and oxidation resistant alloys so as to elfect a furtherimprovement in the thermal fatigue and oxidation resistance thereof.Heat-resistant metals and alloys of the general types to which thecoating composition comprising one aspect of the present invention isapplicable include stainless steels, nickel-base alloys, and theso-called superalloys, such as Hastelloy-X, Inconel 600 and the like.Such heat-resistant alloys are in widespread use in the manufacture ofcomponents for gas turbine engines or the like, in which they areconcurrently subjected to high stresses at elevated temperatures in thepresence of an oxidizing atmosphere which effects a progressivedeterioriation in the physical strength properties thereof, as well as aprogressive oxidation thereof. Protective coatings of the typesheretofore known have either not been particularly effective inproviding a substantial improvement in the corrosion resistance andthermal fatigue characteristics of such heat-resistant alloys, have beenexceedingly difiicult to apply, and/or have been deficient in theiradherence to the substrate in order to form a continuous imperviousbarrier layer which is not brittle and susceptible to fracture orseparation due to shock loading and stresses applied thereto.

There has also been a continuing need for improved high-strengthheat-resistant metal alloys which possess im- Patented June 27, 1972proved thermal fatigue and oxidation resistance enabling such alloys tobe employed directly in high temperature situations without the need ofapplying various protective coatings to the surfaces thereof. Inaccordance with another aspect of the present invention, an improvedcermettype alloy and method for making the alloy is provided which canbe employed in the fabrication of components subject to hightemperatures and stresses such as for components of gas turbines and thelike, providing increased thermal fatigue and oxidation resistance andthereby en- :abling engine operation at higher temperatures, providingfor a substantial improvement in operating efiiciency.

SUMMARY OF THE INVENTION In its composition aspects, the presentinvention is directed to an improved cermet-type alloy either in theform of an ingot or in the form of a protective coating on a substratein which the cermet-type alloy is characterized as comprising asubstantially continuous phase or matrix of a nickel and/or cobalt basealloy containing from about 10% to about 40% chromium, and preferably15% to 25% chromium, along with other conventional impurities andalloying agents in amounts which do not appreciably reduce theheat-resistant properties of the continuous alloy matrix and adiscontinuous phase of precipitated borides and/or silicides of titaniumand/or zirconium which are present in the form of compressed crystalsand are distributed substantially uniformly throughout the continuousmatrix and are present in an amount ranging from about 2% to about 40%,and preferably from about 4% up to about 30% of the alloy.

In its method aspects, the present invention encompasses the formationof the improved cermet-type alloy :by exothermically reacting aparticulated mixture containing the cobalt and/ or nickel-base matrixmetals and a reactive metal selected from the group consisting oftitanium and zirconium and a nonmetallic element selected from the groupconsisting of silicon and boron which are present in controlled amountsso as to effect the formation of the corresponding titanium or zirconiumboride or silicide in situ through an exothermic reaction resulting fromthe heating of the particulated mixture at a temperature at which atleast a partial fusion thereof occurs.

'In the method of forming an ingot of the improved alloy, theparticulated mixture is heated in a suitable refractory crucible or moldin a substantially inert atmosphere to an elevated temperature whichvaries depending upon the specific composition of the mixture butgenerally ranges from about 1900 F. up to about 2100 F. for a period oftime suificient to complete the interaction between the reactive metalsand the nonmetallic elements forming a fused ingot of the cermet-typealloy.

In the method of forming protective coatings on metal substrates, theparticulated mixture is applied to the surface of the metal substrate tobe protected in an amount so as to provide a resultant protective layerhaving an average thickness of from about 0.001 to about 0.010 inch, andpreferably of an average thickness of about 0.002 to about 0.005 inch.It is usually preferred to employ a suitable organic binder forpreliminarily adhering the particulated mixture to the surface of thesubstrate, which subsequently decomposes without residue upon a heatingof the coated substrate in a substantially inert atmosphere such as, forexample, an argon atmosphere or in vacuum at elevated temperatureswithin the range hereinabove set forth, effecting at least partialfusion of the coating and an inter-reaction between the metal reactantsand nonmetallic elements, effecting the formation of the cermet-typealloy coating. The heating of the coating is preferably continued so asto effect a partial diffusion of the continuous metal alloy matrix witha substrate metal, forming therewith a tenacious bond. The protectivecoatings thus formed, upon subsequent cooling, possess a substantiallyhigher remelt temperature which conventionally is in the order of about2100 F. up to about 2350 F. and above, which further enhances theeffectiveness of the protective coating and provides for an improvedcoated article possessing increased resistance against thermal fatigueand oxidation at elevated temperatures.

A further composition aspect of the present invention is directed to anovel particulated composition containing controlled proportions of thematrix metals and reactive constituents which preferably arepre-alloyed, facil itating their handling and increasing theirresistance to oxidation attack during shipment and storage, as well asproviding for lower melting eutectics which reduces the temperature towhich the particulated mixtures must be heated to effect a partialfusion thereof and the initiation of the exothermic reaction. Theparticulated compositions, when intended for use in forming protectivecoatings on metal substrates, are of controlled particle sizes tofacilitate their application in the form of uniform coatings whereasgreater latitude is provided in particle size when such particulatedmixtures are employed for making ingots of the improved cermet-typealloy.

BRIEF DESCRIPTION OF THE DRAWINGS Other advantages and benefits of thepresent invention will become apparent upon a reading of the followingdescription taken in conjunction with the accompanying drawings,wherein:

FIG. 1 is a fragmentary magnified cross sectional view of a sheet of aheat-resistant alloy coated on both surfaces thereof with a particulatemixture temporarily bonded by an organic binder;

FIG. 2 is a magnified fragmentary cross sectional view similar to thatshown in FIG. '1, exemplifying the protective coating after it has beenfused and reacted and tenaciously bonded to the metal substrate;

FIG. 3 is a partly schematic vertical sectional view of a furnacecontaining a crucible in which the powder mixture is exothermicallyreacted forming an ingot of the cermet-type alloy; and

FIG. 4 is a photomicrograph of the metallurgical structure of thecermet-type alloy made in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The proportions of the severalconstituents comprising the particulate mixture applied to a metal alloyto be protected, as well as the composition of the resultant coatingformed, are described in terms of percentages by weight in thespecification and subjoined claims unless expressly indicated otherwise.

In accordance with the practice of the present invention, the improvedcermet-type alloy is obtained by initially forming a powder blend whichcontains, as its essential constituents, the matrix metal consisting ofnickel and/or cobalt in addition to chromium, as well as minorproportions of other conventional impurities present in amounts which donot significantly detract from the high temperature oxidation resistanceand mechanical properties of the matrix metal. The matrix metalconstituent of the powder comprises about 60% to 90%, and preferablyabout 70% up to about 90%, of the powder and wherein the chromiumconstituent comprises from about to 40%, and preferably from about toabout of the nickel and/or cobalt-chromium matrix metals. While cobaltcan be substituted in whole or in part for the nickel in the matrixmetal constituent, the use of nickel itself with cobalt present inamounts up to about 5% constitutes a preferred practice. In addition tothe three matrix metals, namely; nickel, cobalt and chromium, the matrixmetal mixture may also include additional hardening and/or strengtheningagents to provide the desired physical properties of the resultant alloyformed consistent with the ultimate intended use thereof. In thisregard, it is contemplated that iron can be included in the matrix metalmixture in amounts up to about 5%, manganese in amounts usually up toabout 1%, as well as other conventional impurities such as aluminum,carbon, etc.

The balance of the metallic powder consists essentially of at least onereactive metal selected from the group consisting of titanium andzirconium, and at least one nonmetallic element selected from the groupconsisting of silicon and boron, which are effective during the fusionof the powder mixture at an elevated temperature to undergo anexothermic reaction forming the corresponding titanium or zirconiumsilicide or boride. In accordance with the preferred practice, thereactive metal and nonmetallic element or mixtures thereof are presentin stoichiometric proportions so that no residual unreacted reactionmetal and nonmetallic element is present in the resultant cermet-typealloy. It is, however, possible to incorporate quantities of titanium,zirconium and silicon in excess of the stoichiometric proportions suchthat the reaction metal or silicon will be present in an unreacted formin combination with the corresponding silicide in the resultant fusedalloy. It is generally preferred not to employ the nonmetallic boronelement in excess amounts whereby unreacted boron is present in theresultant alloy, although the reaction metals can be present in excessamounts to provide a residuary reaction metal in combination with thecorresponding boride or borides in the final alloy.

The reaction product of the reaction metal and the nonmetallic elementoccurs during the subsequent fusion of the powder blend and isexothermic, forming a nonmetallic ceramic compound or complex which issubstantially insoluble in the nickel/cobalt-chromium alloy metal matrixand is present in the form of precipitated discrete discontinuous phasesdistributed substantially uniformly through the continuous metal alloymatrix. The proportions of the reaction metal and nonmetallic elementare controlled such that the resultant alloy incorporates from about 2%to about 40%, and preferably from about 4% to about 30%, of the ceramicreaction compounds or complexes.

To achieve a resultant alloy containing the reaction compounds and/orcomplexes in an amount from about 2% to 40% of the cermet-type alloy, aswell as the permissible inclusion of small residual unreacted portionsof the zirconium, titanium and silicon constituents, the reactiveportion of the powder mixture generally contains from about 1.5% toabout 28% titanium or 1.8% to about 36% zirconium, and from about 0.5%to about 12% of boron or 0.5% to about 20% silicon. As previouslymentioned, the specific amounts of the boron and/ or silicon present inthe powder mixture within the aforementioned percentages is dictated bythe quantity of the titanium and/or zirconium present and preferably iscontrolled in stoichiometric amounts to form the corresponding silicidesand borides of the reaction metal, as well as complexes thereof.

The manner by which the matrix metals and reactive constituents areintroduced in the powder blend is not critical, although the nonmetallicelement is usually preferably introduced in the form of a pre-alloyedpowder in combination with the nickel/cobalt and chromium matrix metals.The reaction metal similarly is preferably introduced in the form of apre-alloyed powder, preferably in proportions so as to form or approacha lower melting point eutectic of the mixture which reduces the requiredthreshold temperature to which the powder blend must be heated to effecta partial fusion and the initiation of the exothermic reaction. Inaccordance with this preferred practice, a pre-alloyed powder containingof the reaction metal and 30% nickel has been found particularlysatisfactory, as well as pre-alloyed nickel powders containing 30%nickel and 70% titanium or, alternatively, 30% nickel and 70% zirconium.Corresponding prealloyed eutectic combinations of the reaction metalswith cobalt include 30% cobalt and 70% titanium and 15% cobalt and 85%zirconium. The use of such pre-alloyed powders substantially simplifiesthe handling of the reaction metals which are generally susceptible tooxidation attack when in the pure elemental form and ordinarily requirea handling thereof in a nonoxidizing atmosphere. Additionally, aspreviously indicated, the use of such pre-alloyed powders havingcompositions at or near the eutectic of the constituents pro vides alower melting powder mixture enabling the initiation of the exothermicreaction at lower temperatures.

The powder mixture of the several constituents or prealloyed powdersincorporating the reaction metals and the nonmetallic elements in anunreacted condition are 'usually controlled in particle size of fromabout 20 microns up to about 500 microns. In those instances in whichthe powder blend is to be employed for forming an ingot of theoermet-type alloy, greater latitude is provided in the particle sizeenabling use of particles having a size corresponding to maximum size ashereinabove set forth. In such instances in which the powder blend is tobe employed for forming a protective coating on a metal substrate, it isgenerally preferred to control the particle size within a range of fromabout 20 microns up to about 150 microns. 'In the formation ofprotective coatings, it has generally been found that when the particlesize of powder blends is greater than about 150 microns, increaseddifficulty is encountered in applying the powder to a substrate; whereaswhen particles of a size of less than about 20 microns are employed, theattainment of a satisfactory exothermic reaction is inhibited betweenthe reaction metal and the nonmetallic element. It is for this reasonthat when the powder mixture is to be employed for forming a protectivecoating, the particle size is preferably controlled from about 20microns up to about 150 microns. In addition, in the coatingapplication, the particles are preferably randomly distributed oversubstantially the entire particle size range, thereby providing for theadvantages of maximum coating density and improved quality of theresultant cermet-type coating formed.

In one method of the present invention in which a protective coating isformed on a substrate, the powdered blend is applied in the form of auniform layer or in the form of a controlled non-uniform layer, as maybe desired, to all or selected portions of the exposed surface of asubstrate such as a heat-resistant alloy to be protected. Theapplication of the powder mixture to the surface of a heat-resistantalloy can be made by any one of a variety of techniques well known inthe art, and preferably by utilizing an organic binder for adhering theparticles to the surface of the substrate and to retain them in properposition during the fusion operation. For this purpose, any one of avariety of organic binder materals of the types which will thermallydecompose without a residue upon a heating thereof to an elevatedtemperature at which the coating is fused can be satisfactorily employedand include, for example, solutions of plastic materials such aspolyethylene, polypropylene, polyvinyl, polyvinylidene, polyvinyl,alcohol, acrylic resms, such as polymethylmethacrylate, or the like. Theorganic binder can be admixed with the power blend so as to form asuitable slurry, paint or paste, and directly applied to the part to becoated by a spray application, dipping, brushing, flooding or the like.conventionally, it is preferred to form a spray of the organic binder,into which the dry unheated powder particles are injected, forming acomposite spray which is directed against the surface of an object to becoated, forming an adherent substantially uniform film thereon, whichcan be readily handled as is required prior to the fusion operation. Theuse of such organic binder or ancillary bonding agents can be omittedwhen the surfaces to be coated are substantially flat and can beretained in a substantially horizontal position, maintaining uniformityof the particulated coating prior to heat fusion thereof. The specificpowder blend composition of a type suitable for use in accordance withthe practice of the present invention possesses a density such thatcoatings thereof in an amount of milligrams per square inch produce aresultant fused coating of an average thickness of about 0.001 inch.Accordingly, multiples of this coating amount can be employed to produceresultant coatings having average thicknesses of from about 0.001 toabout 0.010 inch thick, as may be desired.

Referring now to FIG. 1 of the drawings, a typical substrate comprisinga sheet 10 of a heat-resistant alloy is illustrated having a coating 12on each face surface thereof, comprising discrete metal particles of thepowder blend retained and adhesively secured to the surfaces of thesheet by an organic binder. The coated sheet, as illustrated in FIG. 1,is subsequently treated at an elevated temperature which conventionallyranges from about 1900 F. up to about 2100 F., at which a partial fusionor melting of the metal particles occurs and during which time theorganic binder thermally decomposes and volatilizes, leaving a residuaryfilm of metal powder on the substrate. During the course of fusion ofthe particulated coating, the exothermic reaction occurs between thereaction metal and the nonrnetallic element and concurrently a diffusionor an alloying of the resultant coating along the shaded areas indicatedat 14 in FIG. 2 occurs adjacent to the surfaces of the sheet 10, formingtherewith a tenacious bond. The resultant fused coating 16 is indicatedas incorporating discrete discontinuous particles of the reactioncomplex, indicated at 18, which are substantially uniformily dispersedthroughout the continuous phase of the nickelcobalt-chromiurn basematrix alloy indicated at 20.

Various heat-resistant alloys have been coated employing the methodcomprising the present invention utilizing powder blends of variantcompositions and have been subjected to oxidation tests and thermalfatigue tests to determine the effectiveness of the coating on theparticular substrate. conventionally, oxidation tests have beenconducted by heating coated samples along with an uncoated sample as acomparative standard in an oven having an air atmosphere at atemperature of from 2100 F. to about 2200 F. The heat cycles have beenconducted for 10 to 20 hour periods, at the end of which the samples areremoved and are bent to determine the exitent of oxidation as indicatedby the brittleness of the entire sheet. Similiarly, the presence ofthermal fatigue is tested by passing the coated samples along with anuncoated comparative standard repeatedly through a flame, effecting arepeated heat-up and cooling thereof; after periodic cycles, the samplesare investigated in the area of heat application to visually determinethe presence of any surface cracks or fractures therein. The foregoingtests clearly establish the improved thermal fatigue resistance andcorrosion resistance of heat-resistant alloys when protected by aselected protective coating applied in accordance with the techniquecomprising the present invention.

The specific benefits of the various coatings are, to some extent,governed by the specific composition of the heatresistant alloy to whichthe powder blend is applied and subsequently fused and bonded. Forexample, the formation of a cermet-type protective coating containing,as its essential constituents, nickel, chromium, and titaniumdisillicide, has been found particularly satisfactory in improving thethermal fatigue resistance of Inconel, while similar coatings containingtitanium diboride provide superior thermal fatigue resistance onHastelloy X in comparison to the titanium silicide-containing coating.On the other hand, the titanium disilicide coating possesses superoroxidation resistance than the corresponding titanium diboride coating.It will be apparent from the foregoing that the specific characteristicsdesired in the resultant coating, that is, thermal fatigue resistanceand oxidation resistance, as well as the specific composition of thetemperature-resistant alloy, will dictate the specific cermet-typecoating to be used so as to maximize the benefits derived thereby.

In another method aspect of the present invention, the powder mixtureincorporating the matrix metal and the reactive constituents can beemployed for forming an ingot or billet of a cement-type alloy whichitself can be employed for fabricating a component satisfactory for useat elevated temperatures where high strength, oxidation resistance andthermal fatigue resistance are requisite characteristics. With referenceto FIG. 3, the powder blend of the desired alloy chemistry and particlesize, as hereinbefore set forth, is placed in a suitable refractorycrucible or mold, indicated at 22, and preferably is lightly compacted,such as by subjecting it to sonic or supersonic vibration, to obtain adensification of the loose free-flowing powder mixture 24. The crucible22 may be formed with a cavity of any desired configuration such thatthe resultant billet or ingot formed of the cermet-type alloy approachesa shape of the desired component to be fabricated therefrom, therebyminimizing final finishing operations. The crucible 22 is thereafterplaced in a suitable furnace 26, which is provided with a non-oxidizingprotective atmosphere, and the powder mixture 24 is heated to anelevated temperature at which a partial fusion and the initiation of theexothermic reaction occurs. At the completion of the exothermicreaction, the resultant fused cermet-type alloy is removed aftersolidification from the crucible and is characterized as in the case ofthe protective coating as consisting of a substantially continuous phaseof the matrix alloy having dispersed substantially uniformlytherethrough a discontinuous discrete phase of precipitated compounds orcomplexes of the reaction metals and nonmetallic elements.

A photomicrograph, as shown in FIG. 4, is illustrative of themicrostructure of a cermet-type alloy made in ac cordance with thepractice of the present invention either in the form of a thinprotective coating or in the form of an ingot. The specific micrographshown in FIG. 4 is taken at a magnification of 120 times and reveals thealloy as having a dense, nonporous cast-type structure and comprised ofa continuous phase indicated at 28 of the matrix metals anddiscontinuous discrete phases comprised of crystals of the reactionconstituents indicated at 30. The specific composition of the powderblend from which the cermet-type alloy shown in FIG. 4 was produced isas follows: chromium, 17.0%; nickel, 66.5%; titanium, 7.6% and silicon,8.9%. The individual discrete phases 30 of the ceramic reactioncompounds and complexes, as noted in FIG. 4, are of an irregular-shapedcrystalline structure formed during the precipitation of the reactionproducts from the molten matrix alloy. The discrete phases 30 exist in acompressed condition within the continuous phase 28, which is a uniquecharacteristic of the cermet-type alloy produced in accordance with themethod aspects of the present invention, and is believed to contributein part to the superior ductility, mechanical strength, oxidationresistance and thermal fatigue properties of the alloy.

In accordance with the practice of the method of forming a cermet-typeprotective coating or ingot as previously described, the heating of thepowder blend to an elevated temperature is achieved in the presence of anon-oxidizing atmosphere to avoid oxidation attack of the powderparticles as they are heated to the fusion temperature. The avoidance ofsignificant contamination of the resultant cermet-type alloy canconveniently be achieved by employing commercially attainable vacuumsor, alternatively, substantially dry inert gases, such as commerciallyavailable argon, which is used to envelop the powder blend either in thecrucible or in the form of a coating on a substrate during its heatingto the elevated temperature at which fusion and the exothermic reactiontakes place.

The particular reaction mechanism and resultant compounds formed duringthe exothermic coreaction between the reaction metal and the nonmetallicelement is not completely understood, it is believed, however, that thereaction occurs so as to form a compound or complex of the twoconstituents which is substantially insoluble in thenickel/cobalt-chromium metal alloy matrix and agglomerates to formprecipitated discrete discontinuous phases dispersed substantiallyuniformly therethrough. In connection with powder blends containing, astheir essential constituents, nickel, chromium, titanium and silicon,the proportion of the titanium and silicon present will, to some extent,determine the specific silicide formed. Thus, when a stoichiometricproportion of one gram atom of titanium is combined with one gram atomof silicon, titanium silicide (TiSi) is formed; whereas, when one gramatom of titanium is combined with two gram atoms of silicon, titaniumdisilicide (TiSi is formed. Titanium trisilicide (Ti Si is formed whenthe equivalent of one gram atom of titanium is reacted with 0.6 gramatoms of silicon. It is believed that the resultant reaction mixturecomprises combinations, as well as complexes, of these ceramic reactionproducts which contribute to the thermal fatigue resistance, oxidationresistance and increased solidus remelt temperature of the resultantalloy and protective coating formed.

The combination of zirconium and silicon forms zirconium silicide(ZrSi), as well as complex compounds, and the stoichiometric proportionis equivalent to one gram atom of each of these constituents.Conventionally, it has been found that the corresponding titaniumsilicides or complexes thereof provide improved protective coatings incomparison to those obtained employing zirconium and silicon. Similarly,the reaction product of one gram atom titanium and two gram atoms ofboron form titanium boride (Tl z), while the coreaction of one gram atomof zirconium and one gram atom of boron produces zirconium boride (ZrB).Generally, the borides of zirconium and titanium have not been found toprovide an improvement in the oxidation resistance, resistance tothermal fatigue and increases in the solidus remelt temperature of theresultant coating of a magnitude equivalent to that achieved by the insitu formation of titanium silicides.

It is also contemplated within the scope of the new alloy and improvedmethod comprising the present invention that in addition to forming theceramic com ponent in situ through an exothermic reaction of thereactive constituents, the in situ formed cermet can be furthersupplemented by the direct addition of titanium nitride (TiN) to theinitial powder mixture in proportions ranging up to about 10%. Theinclusion of the titanium nitride further enhances the oxidationresistance of the alloy and/or protective coating and can beincorporated in the percentages as hereinabove set forth in combinationwith ceramic compounds formed in situ during the exothermic reactionfurther supplementing the total content of the ceramic constituent ofthe cermettype alloy.

In order to further illustrate the present invention, the followingexamples are provided. It will be appreciated that the examples ashereinafter set forth are provided for illustrative purposes and are notintended to be limiting of the scope of this invention as set forth inthe subjoined claims. In the preparation of the protective coatings asset forth in the following examples, pre-alloyed powder mixtures wereemployed containing the several constituents which are admixed to form aresultant particulated mixture, which was applied directly to variousheat-resistant metals substrates. Two prealloyed powders, designated aspowder A and powder B, were employed, respectively, for introducing thetitanium and zirconium reaction metals into the powder mixture. Thecomposition and characteristics of these two prealloyed powders are asfollows:

POWDER A Ingredient: Percent by weight Titanium 70 Nickel 3O POWDER BIngredient: Percent by weight Zirconium 70 Nickel 30 The remainingnickel and chromium, as well as the silicon or boron constituent, areintroduced employing a prealloyed powder designated as C and D,respectively, having nominal compositions as follows:

POWDER C Composition Ingredient: Percent by weight Carbon 0.08

Chromium 18.62 Silicon 0.75 Manganese 0.02 Iron 0.14 Sulfur 0.008Phosphorus 0.007 Boron 0.03 Aluminum 0.01 Titanium 0.03 Zirconium 0.03Cobalt 0.07 Nickel Balance Screen analysis Mesh: Percent +120 Nil-1'20+140 1.5 -140+200 21.6 -200+325 37.7 325 39.0

POWDER D Composition Ingredient: Percent by weight Chromium 15.05 Boron3.53 Carbon 0.03 Nickel Balance Screen analysis Mesh: Percent +120 Nil120+150 4.1 -150+200 24.4 200+325 34.3 325 37.0

Variations in the specific compositions of the powder blends to beapplied to a metal substrate for forming the protective coating weremade by admixing controlled proportions of powder A with powder C toform the corresponding titanium silicide coating; powder A and powder Dto form the corresponding titanium boride protective coating; powder Bwith powder C to form the corresponding zirconium silicide protectivecoating; and powder B with powder D to form the corresponding zirconiumboride protective coating. It will be understood that while the use ofpre-alloyed powders constitutes a preferred technique in accordance withthe method comprising the present invention due to the convenience andsimplicity provided thereby, that alternative owder mixtures also can besatisfactorily employed, including elemental powders which are admixedso as to provide similar proportions of the several constituents of thecoating composition. The use of pre-alloyed powders for introducing thetitanium and zirconium constituents are particularly desirable due tothe reactive nature of these two reaction metals when in a pure andfinely-particulate state, necessitating, in many instances, the use ofinert atmospheres to avoid oxidation attack thereof.

EXAMPLE I A powder mixture was prepared comprising 41.05

grams of powder C and 5 grams of powder A, which corresponds to astoichiometric proportion for the formation of the compound TiSi Theresultant powdered mixture was applied to the surface of a sheet ofInconel 600 employing an organic binder, and thereafter was heated for aperiod of 30 minutes at 2100 F. in a dry argon atmosphere to eifect afusion and exothermic reaction of the constituents thereof. Theresultant protective coating formed was of a gray coarse-grainedappearance and was observed to have excellent resistance to oxidationwhen heated in air to a temperature of 2200 F. The resultant coating hada theoretical composition of 66.5% nickel, 17.0% chromium and 16.5%titanium disilicide.

EXAMPLE II A powder blend was prepared employing a mixture of powder Aand powder D utilizing 9.035 grams of powder D and one gram of powder A,which corresponds to an exact stoichiometric proportion for theformation of the compound TiB without any excess of boron or titanium.The powder mixture was applied to the surface of a sheet of a type 304stainless steel and was heated for a period of 30 minutes at atemperature of 2050 F. in a dry argon atmosphere to produce a well-fusedbrown colored coating. The same powder mixture was applied to thesurface of a sheet of Inconel 600 under the same conditions and wasobserved to produce a fine-grained, blue-gray colored coating. Oxidationtests of both test panels in air when heated to temperatures up to 2100F. did not evidence any oxidation deterioration. The resultant fused andreactive coating composition had a theoretical nominal composition of76.37% nickel, 13.5% chromium and 10.13% titanium diboride.

EXAMPLE III A powder blend was prepared employing twice the quantity ofthe powder D component as used in Example II so as to provide for anexcess of unreacted boron in the resulting cermet-type protectivecoating. The resultant fused coating had 'a nominal theoreticalcomposition of 78.7% nickel, 14.2% chromium, 1.70% boron and 5.4% TiBThe powder blend was applied to the surface of a sheet of Inconel 600and was heated in a dry argon atmosphere for a period of 30 minutes at2050 F. A wellfused thin purple-gray coating was produced having 'aremelt temperature corresponding to the solidus of about 215 0 F.

EXAMPLE IV A powder blend was prepared in which the powder A constituentwas employed in an amount of twice that used in Example II incombination with powder D in order that an excess of titanium is presentin the resultant protective coating. A total of 10 grams of powder D wasadmixed with the two grams of powder A, which, upon subsequent fusionand reaction, resulted in a protective coating having a nominal analysisof 72.8% nickel, 12.5% chromium, 5.4% titanium and 9.3% TiB The coatingmixture was applied in combination with an organic adhesive on thesurface of an Inconel 600 sheet and was heated in a dry argon atmospherefor a period of 30 minutes at 2050 F. A well-fused silver-blue coloredcoating resulted.

EXAM-PLEV A composite titanium boride and titanium silicide protectivecoating was prepared by admixing a pre-alloyed powder, similar to acomposite of powders C and D, having a nominal analysis of 82.94%nickel, 6.5% chromium, 2.5% iron, 3.5% boron and 4.5% silicon, withpowder A so as to produce a resultant coating having a nominaltheoretical analysis of 75.5% nickel, 5.5% chromium, 2.2% iron, 9.7%Ti-B and 7.1% TiS The powder mixture was applied to the surface of asheet of Inconel 600 in combination with an organic adhesive and washeated for a period of 30 minutes at 2050 F. In a dry argon 1 1atmosphere, resulting in a fine-grained thick, blue-gray coloredcoating. The resultant part was heated in air for several hours at 2000F. and the coating produced a viscous, dark-green colored glass-likecoating, which, upon subsequent heating at higher temperaturesculminating in a temperature of 2200 F., produced a well-fused,browncolored coating having good resistance to further oxidation.

EXAMPLE VI A powder blend was prepared comprising a mixture of powder Band powder D in amounts of 9.5 grams and 2.0 grams, respectively,corresponding to an exact stoichiometric proportion necessary for theformation of the compound ZrB without any excess of boron or zirconiumin the resultant fused and reacted coating. The powder blend was appliedto the surface of a sheet of Inconel 600 in combination with an organicbinder and was heated for a period of 30 minutes at 2050 F. In a dryargon atmosphere, producing a fine-grained gray-blue coating. Theresultant coating had a nominal analysis of 72.5% nickel, 12.4% chromiumand 15.1% ZrB The coating was observed to have a remelt temperature of2150 F.

EXAMPLE VII A powder blend was prepared employing 36 grams of powder Dand one gram of powder A, providing therewith an excess of unreactedboron. The nominal composition of the resultant fused coating was 80.1%nickel, 14.6% chromium, 2.6% boron and 2.7% TiB A thin grainy,silver-gray coating was produced upon fusion of the coating for 30minutes at 2050 F. in a dry argon atmosphere.

EXAMPLE VIII A powder blend was prepared comprising 32.88 grams ofpowder C and one gram of powder A, providing there with an excess ofunreacted silicon in the resultant protective coating. The nominalcomposition of the coating formed after fusion for 30 minutes at 2100 F.in a dry argon atmosphere was 69.7% nickel, 18.5% chromium, 7.3% siliconand 4.5% TiSi A fine-grained, gray-brown textured coating was producedwith no signs of oxidation or deterioration after heating in anatmosphere for 150 hours at 2200 F.

EXAMPLE IX A powder blend was prepared containing 16.5 grams of powder Cand one gram of powder A, producing a coating having an excess ofsilicon and a nominal composition of 68.6% nickel, 17.0% chromium, 4.9%silicon and 8.6% TiSi The coating, after fusion for 30 minutes at 2100F. in a dry argon atmosphere, had a brown-gray textured appearance andno signs of oxidation and deterioration were observed after heating inan air atmosphere for 50 hours at 2200 F.

EXAMPLE X A powder blend was prepared by mixing 24.6 grams of powder Cand 10 grams of powder A to produce a resultant coating having a nominalanalysis of 59.2% nickel, 13.5% chromium and 27.3% Ti Si The coating wasap plied, fused and reacted in a manner as previously described. Whenfused in a dry argon atmosphere for a period of 30 minutes at 2150 F., athin fine-grained blue colored coating is produced. This coatingpossesses remelt temperatures exceeding 2350" F.

EXAMPLE XI A powder blend is prepared by admixing powder B with acontrolled proportion of powder C so as to produce a resultantprotective coating having a nominal analysis of 60% nickel, 12.6%chromium and- 27.4% zirconium silicide. This proportion is equal to astoichiometric proportion of one gram atom zirconium and one gram atomof silicon to yield ZrSi.

1 2 EXAMPLE XII A powder blend is prepared comprising a mixture ofpowders A and C in further combination with particulated titaniumnitride in an amount of 8.9% of the resultant mixture. The coating had anominal analysis of 63.8% nickel, 16.9% chromium, 7.0% silicon, 3.4%titanium silicide and 8.9% titanium nitride. This coating possessesexcellent protection against oxidation of a base metal. The applicationand fusion of the powdered mixture is achieved in the same manner aspreviously described, forming a partially diffused adherent protectivecoating.

EXAMPLE XIII A powder mixture is prepared comprising a mixture ofpowders A and C, to which 6% by weight titanium nitride is added, and acoating is formed similar to that as described in Example XII.

EXAMPLE XIV A powder blend is prepared comprising a mixture of powders Aand C, to which 5% by weight of titanium nitride is added and aprotective coating is formed in a manner similar to that as described inExample XII.

EXAMPLE XV Ingots having a nominal weight of about 100 grams wereprepared employing the powder mixtures as described in prior Examples Ithrough XIV which were placed in a refractory crucible within a furnaceprovided with a substantially dry inert argon atmosphere and heated to atemperature for time periods corresponding to the temperature-timeperiods to which the respective coatings were subjected. In eachinstance, a solid ingot of the cermet-type alloy was recovered having aperipheral contour corresponding to that of the crucible andcharacterized by a continuous phase of the matrix metal havinginterspersed therein discrete phases of the ceramic compounds.

In addition to pre-alloyed powders A through D, the following additionalmetallic powders comprising binary alloys of cobalt and nickel with thenonmetallic elements, boron or silicon, and the reactive metals,titanium or zirconium, can also be satisfactorily employed for preparingpowder blends which will exothermically react to form an alloy orcoating in accordance with the practice of the present invention. Thebinary alloys as hereinafter set forth are selected in accordance withthe preferred practice of the present invention in which the specificproportions of the ingredients form lowmelting point eutectics.

POWDER E Ingredient: Percent by weight Boron 4.0 Cobalt 96.0

Melting temperature: 2003 F.

POWDER F Ingredient: Percent by weight Silicon 12.5

Cobalt 87.5

Melting temperature: 2183 F.

POWDER G Ingredient: Percent by weight Titanium 70 Cobalt 30 Meltingtemperature: 1868 F.

POWDER H Ingredient: Percent by Weight Titanium 20 Cobalt Meltingtemperature: 2138 F.

13 POWDER I Ingredient: Percent by weight Boron 3.8 Nickel 96.2 Meltingtemperature: 1076 F.

"POWDER J Ingredient: Percent by weight Boron 12.5 Nickel 87.5 Meltingtemperature: 1782 F.

POWDER K Ingredient: Percent by weight Silicon 11.5 Nickel 88.5 Meltingtemperature: 2057 F.

POWDER L Ingredient: Percent by weight Silicon 29.0 Nickel 81.0 Meltingtemperature: 1767 F.

POWDER M Ingredient: Percent by weight Chromium 44.0 Cobalt 56.0 Meltingtemperature: 2552 F.

POWDER N Ingredient: Percent by weight Zirconium 85.0 Cobalt 15.0Melting temperature: 1798 F.

The following additional examples are provided to illustrate powderblends comprising mixtures of selected ones of the foregoing pre-alloyedpowders which can also be satisfactorily employed in forming cermet-typealloys and coatings in accordance with the practice of the presentinvention.

EXAMPLE XVI A powder composition is prepared containing 100 grams ofpowder E, 12.65 grams powder G and 37.5 grams of powder M. Thiscomposition contains stoichiometric proportions of the titanium andboron reactive constituents to form the compound TiB The resultantcermet-type alloy formed, upon fusion of the powder blend at an elevatedtemperature, comprises a substantially continuous matrix of the metalscobalt and chromium with the titanium diboride compound interspersedsubstantially uniformly therethrough as discrete phases. The compositionof the resultant alloy is nominally 80.45% cobalt, 11.0% chromium and8.55% titanium diboride.

EXAMPLE XVII A powder blend is prepared comprising a mixture of 90.35grams of powder D and grams of powder G. The proportions of the severalconstituents provide a continuous metal matrix consisting of nickel,chromium and cobalt and the titanium and boron reactive constituents arepresent in substantially stoichiometric proportions to form the compoundTiB The fusion of the powder blend at an elevated temperature produces acermet-type alloy having a theoretical composition of 73.25% nickel,13.55% chromium, 3% cobalt and 10.2% titanium diboride.

14 EXAMPLE XVIII A powder blend suitable for forming a cermet-type ingotor coating is prepared by mixing grams of powder F, 15.22 grams ofpowder G and 50 grams of powder M. The proportions of the ingredientspresent provide for a continuous metallic matrix composed of cobalt andchromium and the reactive titanium and silicon constituents are presentin substantially stoichiometric proportions to form the compound TiSiThe resulting alloy, after fusion at an elevated temperature, has atheoretical composition of 72.7% cobalt, 13.3% chromium and 14.0%titanium disilicide.

EXAMPLE XIX A powder blend is prepared to provide a resultantcermet-type alloy in which the matrix metals are nickel, cobalt andchromium by mixing 82.1 grams of powder C and 10 grams of powder G. Thetitanium and silicon reactive constituents are present in substantiallystoichiometric proportions to form the compound TiSi Upon exothermicreaction, the resultant cermet-type alloy in the form of an ingot orcoating has a theoretical composition of 63.5% nickel, 16.6% chromium,3.3% cobalt, 16.1% titanium disilicide, 0.2% titanium and 0.2% iron.

EXAMPLE XX A powder blend is prepared by mixing 100 grams of powder F,19.84 grams of powder N and 50 grams of powder M, the zirconium andboron reactive constituents are present in substantially stoichiometricproportions to form the compound ZrB The substantially continuousmetallic matrix consists of an alloy of cobalt and chromium. Theresultant cermet-type alloy produced upon fusion and reaction at anelevated temperature has a theoretical composition of 74.75% cobalt,12.95% chromium and 12.3% zirconium diboride.

Alternative mixtures of powders A through N can be prepared in furthercombination with elemental powders of the several constituents toprovide reactive powder mixtures and resultant cermet-type alloys havingcompositions within the specific proportions as hereinbefore set forth.

While it will be apparent that the description of the preferredembodiments and the specific examples disclosed are well calculated toachieve the advantages and benefits of the present invention, it will beappreciated that the invention is susceptible to modification, variationand change without departing from the spirit of the invention.

What is claimed is:

1. A metal article having a dense, non-porous cermettype protectivecoating on at least a portion of the surface thereof, said coatingcomprising about 60% to about 98% of a continuous metallic matrixconsisting essentially of from about 10% to about 40% chromium and thebalance comprising at least one metal selected from the group consistingof nickel and cobalt, said metallic matrix having interspersedtherethrough discrete discontinuous phases in compressed conditionconsisting essentially of compounds and complexes of intermetallicreaction products selected from the group consisting of titaniumsilicides, titanium borides, zirconium silicides, zirconium borides, andmixtures thereof, said intermetallc reaction products formed during anin situ exothermic reaction of at least one reaction metal selected fromthe group consisting of titanium and zirconium and at least onenonmetallic element selected from the group consisting of silicon andboron in an inert atmosphere in the presence of metals of which saidmetallic matrix is comprised, said discontinuous phase comprising fromabout 2% to about 40% of said protective coating, said coatingtenaciously bonded to and alloyed with said article at the interfacetherebetween.

16 2. The metal article as defined in claim 1 in which References Citedsaid continuous metallic matrix comprises from about UNITED STATESPATENTS 70% to about 96% of said coating and said discontinu- Ous phasescomprise from about 4% to about 30% of 3,023,490 3/1962 Dawson 29194said coating. 5 3,291,577 12/1966 Davies et a1 29-1822 3. The metalarticle as defined in claim 2, in which 3,421,890 1/1969 Bfiumel 75-171said continuous metallic matrix consists essentially of about 15% toabout 25% chromium, up to about 5% DEWAYNE RUTLEDGE Pnmary Exammercobalt, and the balance nickel. E. L. WEISE, Assistant Examiner

